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Mr. Johnson, though, countered that passing a legalization bill would actually enhance security because it would get illegal immigrants to come forward and report, and would allow the department to more narrowly focus its resources.

In one key step, the Obama administration this year is calling for a cut in detention beds that hold immigrants awaiting deportation, from the 34,000 level mandated by law to slightly more than 30,500.

“We believe that is not the best and highest use of our resources, given our current estimates of who we need to detain, who we regard as public safety, national security, border security threats,” Mr. Johnson said.

Homeland Security officials argue that rather than holding illegal immigrants in detention facilities, they can be given supervised release, which he said is more cost-effective.